I happen to be typing this from a Dutch Hotel.
This particular one has free wireless, and there is no way to identify a particular system accessing the net to a room. In fact, without staying here I could still probably sit in the car park or hotel lobby and access the internet from there. There's even a PC in the lobby with anonymous access from it.
Granted it does use a "Hotspot" login page (just need to check a checkbox and click login), so I suppose that could be modified to have someone provide a room number or PIN etc...
Changing the way things work though will invariably be a pain though, especially if you need to access the Internet over the weekends and the authentication system breaks down or something else goes wrong... (as seems to be quite common with the systems in many hotels). Reception tend to look at you with rather blank faces when this happens, and it usually isn't fixed until a weekday.